1. Field of the Invention
The invention relates to a hydraulic control apparatus that switches speed change steps of a vehicle automatic transmission.
2. Description of the Related Art
There have been proposed various hydraulic control apparatuses that switch speed change steps of an automatic transmission that constitutes a portion of a power transmission path of a vehicle, by releasing or engaging engagement elements of the automatic transmission. An example of such hydraulic control apparatuses is a hydraulic control apparatus for a vehicle automatic transmission that is disclosed in Japanese Patent Application Publication No. 2004-76926 (JP-A-2004-76926). The vehicle automatic transmission described in the publication No. 2004-76926 includes a first clutch as one of the aforementioned engagement elements. In order to hydraulically control the release or engagement of the first clutch, the hydraulic control apparatus is equipped with a D-range pressure channel that is supplied with a predetermined D-range pressure when a selector device is operated to a forward-travel position, as well as with a check valve, and a pressure control valve capable of continuously changing the oil pressure at its output port.
The output port of the pressure control valve is connected to the first clutch, and an input port of the pressure control valve is connected to the D-range pressure channel. Besides, the check valve is provided parallel to the pressure control valve, between the first clutch and the D-range pressure channel. While allowing pressure oil to flow from the first clutch into the D-range pressure channel, the check valve blocks the pressure oil to flow in the opposite direction.
Due to this construction, the hydraulic control apparatus is able to quickly discharge the pressure oil from the first clutch via the check valve, for example, when the selector device is operated from the forward-travel position to another position while the first clutch is in an engaged state.
On the basis of the hydraulic control apparatus shown in the publication No. 2004-76926, a construction is assumed in which the pressure control valve is replaced with a switching valve that is capable of blocking the engagement oil pressure of the first clutch for fail-safety. In that case, the input port of the switching valve is supplied with a regulated engagement oil pressure. In this construction, since the check valve is provided parallel to the switching valve so as to permit the discharge of the pressure oil from the first clutch, the pressure oil of the first clutch is discharged via the check valve at the time of release of the first clutch, and the position of the switching valve is not constrained. However, the pressure oil is discharged, via the check valve, directly to the D-range pressure channel whose pressure is made equal to a drain pressure. Therefore, there is a problem of not being able to control the oil pressure at the time of release of the first clutch (engagement element) through the use of the pressure control valve. Therefore, at the time of release of the first clutch, there is a risk of the releasing operation of the engagement element becoming instantaneous and therefore the shift shock becoming large. Incidentally, this problem is not known to public.